Quaternary ammonium compounds, such as didecyldimethyl ammonium carbonate and didecyldimethyl ammonium chloride, are known to have antimicrobial activity. See, for example, U.S. Pat. Nos. 5,523,487, 5,833,741, and 6,080,789. Quaternary ammonium compounds have been found to be particularly useful as wood preservatives. However, quaternary ammonium chloride have been found to leach rapidly in soil (Nicholas et al., Forest Prod. J., 41:41 (1991)). Consequently, a metal coupler, such as a copper salt, is frequently added to the quaternary ammonium chlorides to prevent leaching.
Quaternary ammonium carbonates, on the other hand, have better leaching resistance and do not require the use of a metal coupler. As a result, there is an increasing demand in the preservative market for quaternary ammonium carbonates.
U.S. Pat. No. 5,438,034 discloses a process for preparing quaternary ammonium carbonates. The process includes reacting a quaternary ammonium chloride with a metal hydroxide to form a quaternary ammonium hydroxide and then reacting the quaternary ammonium hydroxide with carbon dioxide to yield the quaternary ammonium carbonate. The quaternary ammonium hydroxide, however, is very corrosive. In addition, metal chloride produced as a byproduct in the first reaction must be filtered out of the reaction product, a step which increases the cost and decreases the efficiency of the process. Thus, an alternative method to produce quaternary ammonium carbonates is desirable.
Werntz, U.S. Pat. No. 2,635,100, discloses a process for preparing quaternary ammonium carbonates by reacting a trialiphatic amine with a dialiphatic hydrocarbon ester of carbonic acid, such as dimethyl carbonate and ethylene carbonate, preferably in the presence of an alcohol. Werntz reported that the reaction of tertiary amines and dimethyl carbonate yielded quaternary ammonium methocarbonates. Werntz also reported that when ethylene carbonate was reacted with triethylamine and methanol, the cyclic carbonate of triethyl-2-hydroxyethylammonium hydroxide was formed. The solvent, unreacted amine, and cyclic ester were removed by distillation. Many dialiphatic hydrocarbon esters of carbonic acid, such as dimethyl carbonate, are expensive and, therefore, significantly increase the cost of preparing quaternary ammonium carbonates by this process.
Dimethyl carbonate is commercially available, and methods of its synthesis are well known in the art. Typically cyclic carbonates, e.g., ethylene and propylene carbonate, are converted to dimethyl carbonate and a glycol in the presence of methanol or other alcohol and catalyst. Romano et al., U.S. Pat. No. 4,062,884, disclose a process for preparing dialkylcarbonates by reacting an alcohol with a cyclic carbonate in the presence of an organic base, such as a tertiary aliphatic amine. Romano et al. describe the reaction of a methanol/ethylene carbonate/triethylamine mixture. Continuous distillation of the methanol-dimethylcarbonate azeotrope over 3 hours resulted in almost complete conversion of ethylene carbonate to ethylene glycol and dimethyl carbonate. Romano et al. further teach that the organic base which catalyzes the reaction can be totally recovered from the reaction vessel by simple distillation.
There is a continuing need for cheaper and more efficient methods for preparing quaternary ammonium carbonates. A one step, in situ method of preparing quaternary ammonium carbonates would advantageously meet these needs.